In order to produce a fibrous sheet material it is essential to obtain uniform distribution of fibrous material in a gas stream with a predetermined dispersity, and to maintain this dispersity along the entire flow path of the gas-fiber stream.
The gas-fiber stream must possess specific fluidity enabling its shape to be transformed to a flat stream, as well as an internal structure capable of achieving a homogeneous gas-fiber distribution throughout the stream.
The dispersity value of the air-fiber stream is assumed to be a ratio of the volume of discrete fibers or small fibrous aggregates to the volume of an individual fiber of mode-length, i.e. the length which predominates in the fiber length distribution.
The homogeneous fiber distribution in the air stream assumes the fiber concentration in each individual stream volume to have little or no fluctuation.
The dispersity value of the air-fiber stream and the homogeneous fiber distribution throughout the stream determines the degree of uniformity and structural homogeneity of the obtained fiber sheet material. The degree of the fiber concentration in the air stream determines the amount of gas to be removed from the air-fiber mixture to form a layer of fibrous material on a flat screen.
The dispersity value of the air-fiber suspension is reduced by high autoadhesion of the fibers, i.e. clustering of separate fibers takes place. To decrease the likelihood of fiber collision causing clustering due to turbulent forces induced in the moving air-fiber stream, the fiber concentration must be low. Generally, the fiber concentration should be in the range from 5 to 30 g/m.sup.3 depending upon the properties of the material produced and the kind of fiber.
Moreover, high fiber concentration in the air-fiber stream decreases the stream fluidity. Fluidity is a prerequisite for transforming the outer shape of the air-fiber stream, e.g. a cylindrical shape into a flat one, as well as for changing the internal structure of the air-fiber stream to attain uniform distribution of the velocity field in the stream cross-section, this being necessary for forming a uniform layer of fibrous material on a flat screen.
Thus, low fiber concentration of the gas-fiber stream is a necessary prerequisite for forming a fibrous sheet material. Therefore, if a layer of fibrous material is being formed with high velocity, e.g. on the flat screen traveling at a velocity ranging from 180 to 900 m/min, a considerable amount of gas is to be removed from the gas-fiber mixture.
The flat screen with fibers settled thereon to form a layer of fibrous material has a high resistance coefficient value of from 20 to 500, depending on the kind of fiber and on the fibrous layer thickness. Therefore, the removal of a large amount of gas per unit of time, required in high-speed manufacturing of the fibrous layer, leads to increased electric power consumption.
The power expended in overcoming the resistance can be reduced with an adequate increase in an active area of the flat screen. This leads, however, to an objectionable increase in the size of the equipment.
The power consumed in overcoming the resistance developed on the screen during a layer forming process, when gas is being removed through the screen and fibrous layer precipitated thereon, can be reduced by increasing the concentration of fibers in the gas-fiber stream. In this manner the gas-fiber stream must be expanded before it is supplied onto the screen to achieve uniform distribution of the velocity field, homogeneous distribution of fibers over the entire stream volume, and an increase in the dispersity value of the gas-fiber stream. All these factors make it possible to obtain a layer of fibrous material of homogeneous structure.
Known in the art is a method for producing fibrous sheet material (cf. U.S. Pat. No. 2,689,985). According to this method the fibrous material is finely divided and is delivered into the expanding gas stream to be transformed therein by mechanical intermixing, whereby a uniform distribution of the velocity field is achieved and separation of large aggregates into small fibrous solids takes place. The gas-fiber mixture then precipitates on the screen to form a fibrous layer thereon.
A device for carrying out this method for producing fibrous sheet material comprises a disc mill to individualize the fibers, said mill being connected through a discharging pipe to a diffuser having diverging side and frontal walls, and a rotating roller arranged therein, the latter comprising teeth. The gas-fiber stream is transformed with mechanical agitation by means of the rotating roller, thus resulting in homogeneous fiber distribution throughout the entire volume of the gas-fiber stream. The gas-fiber stream is supplied from the diffuser onto a flat screen.
Mounted under the screen is a suction box for removing gas from the gas-fiber stream supplied onto the screen during the layer forming process.
The disadvantage of the above-mentioned apparatus is that because of local fiber flocculation caused by mechanical agitation, it only enables a gas-fiber stream having a fiber concentration as low as 5 to 10 g/m.sup.3 to be transformed. When a gas-fiber stream of higher concentration is used, homogeneous distribution of fibers throughout the entire volume of the stream is disturbed.
Furthermore, the gas-fiber stream supplied onto the flat screen has a low fiber concentration. This leads to increased electric power consumption to overcome the resistance developed on the flat screen during the fibrous layer forming process when large amounts of gas are being removed through the screen and fibrous layer settled thereon.
A gas-fiber stream of higher fiber concentration can be supplied onto the screen if the fibers are thoroughly dispersed in the gas before being supplied onto the flat screen.
Known in the art is another method for producing fibrous sheet material. In this process, the fibrous material is ground and fed into the expanding gas stream. The obtained gas-fiber stream is thoroughly dispersed and supplied onto the flat screen to form a fibrous layer thereon.
An apparatus for carrying out this process comprises a diffuser having diverging side walls, with its inlet opening communicating with a gas-fiber stream supply pipe-line, and its outlet opening connected to a rectangular upright chamber. Several airfoils are arranged in the chamber with their planes parallel to the chamber side walls, with the upper portion of each body dispersed inside the diffuser.
When the gas-fiber stream impacts against the airfoil, thorough dispersion occurs due to resilient repulsion of the fibers against the convex surfaces of the airfoils whereby the gas-fiber stream is distributed uniformly edge-wise over the rectangular chamber, and uniform distribution of the velocity field is attained.
The above-mentioned method and apparatus for producing fibrous sheet material, however, fails to transform a gas-fiber stream having fiber concentrations higher than 5 to 15 g/m.sup.3. If a gas-fiber stream of higher fiber concentration is fed to airfoils, the power of the distribution field is insufficient to intermix the gas-fiber stream containing a large quantity of fibers per unit. As a result, a uniform velocity field distribution is not achieved in the transformed gas-fiber stream.
Moreover, the fiber concentration of the stream supplied onto the flat screen continues to be low, leading to an increased consumption of power to overcome the resistance developed on the flat screen during the fibrous layer forming process, since large amounts of gas must be removed through the screen and the fibrous layer precipitated thereon.
The gas-fiber stream can be transformed, simultaneously using the multiple fiber dispersion effect and transversal pulsations induced in the gas-fiber stream.
Known in the art is another method for producing fibrous sheet material. In this process, fibers are dispersed in a gas stream to obtain a gas-fiber stream, which is distributed in a flattened form. The flattened gas-fiber stream is transformed by supplying it to a cylinder element.
The interaction of the flattened gas-fiber stream with the cylinder causes thorough fiber dispersion resulting from the resilient impingement of fibrous solids against the cylinder surface. Thus solid fiber shredding, i.e. increasing the dispersion value, takes place.
The cylinder, scheduled in the apparatus, provides for transversal pulsations in the gas-fiber stream flowing over the cylinder, resulting in uniform distribution of the stream velocity field. The transformed gas-fiber stream is then supplied onto the flat screen to form a fibrous layer thereon. The fibrous layer is subjected to subsequent treatment to obtain a finished sheet material.
A device for carrying out the above-mentioned process for producing fibrous sheet material comprises an elongated slot nozzle having mutually perpendicular diverging side walls and converging frontal walls. The inlet opening of said nozzle communicates with means for dispersing fibers in a gas stream to obtain a gas-fiber mixture, while its outlet opening communicates with a chamber.
Arranged underneath the elongated nozzle, along its entire length, is a cylinder with its ends affixed to the side walls of the chamber. A special lattice for eliminating stream turbulence is placed downstream from the cylinder and spans the chamber cross-section.
The layer forming process takes place on a flat screen with the help of a suction box disposed under the chamber.
The disadvantages of the aforesaid apparatus for producing fibrous sheet material are that this apparatus can only transform a gas-fiber stream having a fiber concentration of 10 to 30 g/m.sup.3. When the gas-fiber stream is delivered onto the cylinder, transversal pulsations in the gas-fiber stream flowing past the cylinder are generated, with the power of said pulsations gradually decreasing as the stream moves away from the cylinder. Therefore, when a gas-fiber stream of higher fiber concentration is fed to the cylinder, the power of the distribution fields and the transversal pulsations are insufficient for transforming the stream and for obtaining a sheet material homogeneous in structure.
None of the stream transforming apparatus can provide the desired degree of transforming a stream having a high fiber concentration.
Consequently, only a gas-fiber stream having low fiber concentration can be supplied on the flat screen in order to produce sheet material homogeneous in structure. This results in increased power consumption, since a large amount of gas must be removed per unit of time during the layer forming process.